The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with in-vivo examinations and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in conjunction with the magnetic resonance spectroscopic examination of localized regions for imaging, chemical shift analysis, and the like.
In non-imaging magnetic resonance spectroscopy, samples in a test tube may be immersed or dissolved in a solvent, such as water, before undergoing analysis. When analysing a sample for chemical compositions containing hydrogen, magnetic resonance signals from a water solvent are often several magnitudes greater than the magnetic resonance signals from the composition of interest. Similarly, signals from solvents other than water often drown out the signals of interest.
One technique for separating the solvent signals from the signals attributable to the composition of interest includes the use of binomial magnetic resonance excitation pulses. As set forth in "Solvent Suppression in Fourier Transform Nuclear Magnetic Resonance," J. Mag. Res., Vol. 55, pages 283-300 (1983), P. J. Hore, the binomial pulses selectively excite nuclei within limited ranges of resonance frequencies relative to the main magnetic field. That is, the binomial pulses excite magnetic resonance in specific, limited frequency ranges and suppress magnetic resonance excitation in other resonance frequency ranges. In this manner, resonance can be selectively excited in the nuclei of interest while suppressing resonance signals from the solvent.
Heretofore, the use of binomial pulses has been limited to chemical shift spectroscopy. In magnetic resonance imaging, gradient pulses are commonly applied concurrently with the excitation pulses. The gradients across the main magnetic field cause nuclei to have different resonance frequencies at different spatial positions along the gradient. The application of a binomial excitation pulse concurrently with a magnetic field gradient would shift the excitation frquency bands spatially such that the range of excited frequencies is different at each spatial position. The spatial shift would undo the selective excitation of only specific chemical compositions to the exclusion of others.
Also, in the field of chemical shift spectroscopy, various techniques have been developed to localize the examined region, such as the technique illustrated in U.S. Pat. No. 4,480,228 issued Oct. 30, 1984 to P. A. Bottomley. In the Bottomley patent, a 90.degree. magnetic resonance excitation pulse is applied in the presence of a positive z-axis gradient. A first 180.degree. inversion pulse is applied in the presence of a y-axis magnetic field gradient which causes a first spin echo. Subsequent to the first spin echo, a second 180.degree. inversion pulse is applied in the presence of an x-axis magnetic field gradient. The second 180.degree. pulse refocuses only the magnetization of resonating nuclei at the intersection of the three gradient planes. In this manner, only nuclei at the intersection of the planes contribute to a second spin echo. Chemical shift data acquired during the second spin echo is descriptive only of the nuclei at the intersection.
The Bottomley selective spatial localization technique is not amenable to solvent suppression. The application of the necessary gradient magnetic field concurrent with the excitation pulse renders the Bottomley technique incompatible with the Hore technique.
The present invention provides a new and improved technique which provides both solvent suppression and spatial region localization which is suitable for both chemical shift and imaging examinations.